noglob wip
archive: convert pathspec to 'struct pathspec' sooner

We store a "const char **" in our archiver_args, but then
convert it to a "struct pathspec" at point of use. Let's
just move to the final form immediately, which will open up
more room for refactoring pathspecs.
run_add_interactive: use argv_array

This makes the code a little simpler, as we do not have to
worry about pre-counting the pathspecs, nor about the magic
"5" to leave room for other arguments.
cleaning up add's pathspec usage WIP

Ugh, this is revealing just how many places in the code
really want to treat pathspecs as a NULL-terminated array of
strings.